N-Heterocyclic carbenes (NHCs) are unique and enabling ligands for transition metal (TM) catalysis and have emerged as selective organocatalysts for a remarkably diverse array of transformations (Arduengo, A. J., Acc. Chem. Res. 1999, 32, 913-921; Herrmann, W. A., Angew. Chem., Int. Ed. 2002, 41, 1290-1309; Cesar, V. et al., Chem. Soc. Rev. 2004, 33, 619-636; Marion, N. et al., Angew. Chem., Int. Ed. 2007, 46, 2988-3000; Hahn, F. E. et al., Angew. Chem., Int. Ed. 2008, 47, 3122-3172; Enders, D. et al., Chem. Rev. 2007, 107, 5606-5655; Bugaut, X. et al., Chem. Soc. Rev. 2012, 41, 3511-3522; Cohen, D. T. et al., Chem. Sci. 2012, 3, 53-57; Douglas, J. et al., Synthesis 2012, 44, 2295-2309; Vora, H. U. et al., Adv. Synth. Catal. 2012, 354, 1617-1639; De Sarkar, S. et al., Chem. Eur. J. 2013, 19, 4664-4678; Ryan, S. J. et al., Chem. Soc. Rev. 2013, 42, 4906-4917; Phillips, E. M. et al., J. Am. Chem. Soc. 2010, 132, 13179-13181; Candish, L. et al., Chem. Sci. 2012, 3, 380-383; Chen, J.; Huang, Y. Nat. Commun. 2014, 5). Given the importance of these strongly nucleophilic Lewis bases in chemistry, major efforts by many investigators have produced numerous classes of structurally and electronically diverse N-heterocyclic carbenes. Within this truly broad family of heteroatom-stabilized divalent carbon species, two major classes of N-heterocyclic carbenes derived from triaziolium and imidazolium salts have demonstrated applicability as ligands for transition metal and Lewis base catalysis. Traizolium salts have seen broad application in asymmetric organocatalysis, but their success in metal-based transformations has been limited. In contrast, imidazolium-derived N-heterocyclic carbenes (imidazol-2-ylidenes) have been widely deployed as successful ligands for transition metal catalysis and are unique catalysts for organocatalytic transformations. The imidazol-2-ylidene, IMes, was first introduced by Arduengo in 1992 (Arduengo, A. J. et al., M. J. Am. Chem. Soc. 1992, 114, 5530-5534. Surprisingly, since the disclosure of this important species over two decades ago there still remain few chiral scaffolds based on IMes (Bolm, C. et al., Organometallics 2002, 21, 707-710; Broggini, D. et al., Helv. Chim. Acta 2002, 85, 2518-2522; Perry, M. C. et al., Tetrahedron: Asymmetry 2003, 14, 951-961; Seo, H.; Park, H.-j.; Kim, B. Y. et al., Organometallics 2003, 22, 618-620; Fürstner, A. et al., J. Am. Chem. Soc. 2007, 129, 12676-12677; Matsuoka, Y. et al., Chem. Eur. J. 2008, 14, 9215-9222; Struble, J. R. et al., Org. Lett. 2008, 10, 957-960; Würtz, S. et al., J. Am. Chem. Soc. 2009, 131, 8344-8345; Ma, Q. et al., Tetrahedron: Asymmetry 2010, 21, 292-298). The saturated analog of IMes (4,5-Dihydro-1,3-dimesityl-1H-imidazolium, SIMes) has been translated into multiple chiral variants. Although these chiral SIMes N-heterocyclic carbenes have been successful in transition metal catalysis, their reactivity in organocatalysis is markedly different from IMes and have only recently been employed in asymmetric transformations.
A major challenge in carbene catalysis is the design and implementation of a chiral IMes analog with competent ligand and/or catalyst characteristics. While C2-symmetric chiral imidazoliums are presumably the most accessible through the dimerization of stereodefined amines, these N-heterocyclic carbenes often deliver low levels of selectivity as ligands in transition metal catalysis and are typically unsuitable as organocatalysts (Herrmann, W. A. et al., Angew. Chem., Int. Ed. 1996, 35, 2805-2807; Herrmann, W. A. et al., Organometallics 1997, 16, 2472-2477). Most notably, structurally rigid N-heterocyclic carbenes that invoke planar chirality are scarce in the literature, with most contemporary examples featuring pendant planar chiral motifs with varying degrees of free rotation.
The success of planar chiral ligands and catalysts in asymmetric catalysis led to creating a new class of structurally rigid planar chiral N-heterocyclic carbenes (Bolm, C. et al., Chem. Soc. Rev. 1999, 28, 51-59; Matsushima, Y. et al., J. Am. Chem. Soc. 2001, 123, 10405-10406; Gibson, S. E. et al., Org. Biomol. Chem. 2003, 1, 1256-1269). Ferrocenyl-based motifs have arguably received the most attention as planar chiral scaffolds due to their successful application in imparting high levels of selectivity in asymmetric catalysis (Halterman, R. L. Chem. Rev. 1992, 92, 965-994; Togni, A. Angew. Chem., Int. Ed. 1996, 35, 1475-1477; Richards, C. J. et al., Tetrahedron: Asymmetry 1998, 9, 2377-2407; Colacot, T. J. Chem. Rev. 2003, 103, 3101-3118; Fu, G. C. Acc. Chem. Res. 2004, 37, 542-547; Gómez Arrayás, R. et al., Angew. Chem., Int. Ed. 2006, 45, 7674-7715). Encouraged by the high levels of facial selectivity conferred by these scaffolds, it is desirable to create a N-heterocyclic carbene featuring fusion of a metal sandwich complex with the core structure of an N-heterocyclic carbene framework.